1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to a rechargeable alkali metal electrochemical cell, particularly a lithium-ion secondary cell, designed for use in the vicinity of a Magnetic Resonance Imaging (MRI) system.
2. Prior Art
Lithium secondary cells have been under development for many years. Early efforts focused on the use of a lithium anode coupled with metal oxide and metal sulfide cathode materials such as manganese dioxide, titanium disulfide, and others. Despite the enormous amount of research performed on lithium secondary systems, they have not found widespread commercial use. Of concern are the inherent safety problems associated with them. During use, lithium plating can occur in an undesirable manner with dendritic lithium penetrating through the separator and short circuiting the cell. In addition to rendering the cell inoperative, this condition can cause the cell to vent or, in extreme cases, to explode.
During the past decade, increased attention has focused on the use of electrode materials which are capable of more effectively intercalating and de-intercalating lithium ions than the previously used metal oxides and metal sulfides. Cells incorporating such second generation electrode materials are typically referred to as lithium-ion or lithium-rocking chair systems. Although the energy density of these secondary cells is lower than that of primary cells containing lithium metal anodes, they exhibit a higher open circuit voltage, an acceptably high operating voltage and, in many cases, equivalent or better rate capability than many previously developed lithium secondary systems. Most importantly, their safety is generally accepted to be much better.
Presently, lithium-ion secondary cells are used in a large number of commercial applications including telephones, camcorders and other portable electronic equipment. They have been made in a variety of shapes, sizes and configurations including coin, button, cylindrical and prismatic cells. There are several other applications, however, for which rechargeable lithium cells and, in particular, lithium-ion secondary cells may be used but for which present day constructions are unsuitable. Such applications include medical instruments, implantable medical devices and surgical tools.
For many of these applications, the use of prior art lithium-ion secondary cells is unacceptable due to their shape and construction. For instance, modern technology enables surgeons to operate within the confines of an MRI system. One requirement for using battery powered surgical instruments and tools within or near this equipment is that they have as low a magnetic susceptibility as possible so as not to distort the visual image provided by the MRI system to the surgeon. Because present day lithium-ion cells are constructed using a nickel plated steel case, they are too magnetic; therefore, they cannot be used in the vicinity of an MRI system. Also, many implantable devices such as cardiac defibrillators, left-ventricular assist devices, implantable neurostimulators and the like require the use of a hermetically sealed power source. Present day lithium-ion cells are not hermetically sealed but are crimp sealed, precluding their use in these types of applications. Furthermore, in certain types of medical applications, prismatic cells which are sized and shaped for use within the human body are most preferred.
Accordingly, there exists the need for lithium-ion secondary cells which are, among other things, hermetic, have a shape suitable for use with medical instruments, implantable medical devices, surgical tools and the like, and, preferably, have a magnetic susceptibility that makes them acceptable for use in the vicinity of an MRI system.